1. Field of the Invention
The present invention relates to an apparatus for controlling a brushless motor, and more particularly to a motor-controlling apparatus made in the form of a semiconductor integrated circuit.
2. Description of the Related Art
Among motors used in small-sized tape recorders is a Hall motor. This motor can be controlled by various control devices, one of which is a Hall motor-controlling circuit which has a Hall element. The Hall motor-controlling circuit is located such that the Hall element is placed near the rotor of the Hall motor and detects the position of the rotor.
Let us assume that the Hall motor-controlling circuit supplies two drive pulse signals, out of phase and having a 180.degree. C. duration, to the first ends of the two coils of the motor, respectively, whilst a predetermined voltage is applied between the second ends of the motor coils. In this case, the moment either drive pulse signal falls from a high level to a low level, a reverse voltage is generated in the first end of either coil due to the inductance of the coil. The reverse voltage is about twice the predetermined voltage applied between the second ends of the motor coils. It follows that the amplifier connected to the output of this motor-controlling circuit needs to withstand a voltage twice as high as the predetermined voltage. To make matters worse, the reverse voltage disable the circuit to output a signal whose level changes smoothly, inevitably resulting in rotation noise of the motor.
In the case of a Hall motor-controlling circuit made in the form of an integrated circuit, the sizes of its components are proportional to the withstand voltage which the circuit needs to have. Obviously, the larger the components, the difficult it is to increase the integration density of the circuit. On the other hand, in order to increase the withstand voltage of the circuit, the structure of the circuit must be complex. The more complex the structure, the more steps must be performed to manufacture the circuit, inevitably increasing the manufacturing cost of the circuit.
Let us assume that the Hall motor-controlling circuit described above is incorporated in a tape recorder and controls the motor used therein. In this case, the speed of rotation of the motor is maximal when no torque load is applied on it, and gradually decreases as the torque load on it increases during the tape-feeding operation. In other words, the speed of rotation of the motor changes with the load applied on the motor.
The conventional Hall motor-controlling circuit changes the speed of rotation of a motor, form a first predetermined value to a second value, by either of two alternative methods. The first method is to use an additional motor designed to rotate at the second speed. The second method is to change the drive voltage applied to the motor coil of the motor. Either method involves the use of additional components, and leads to a great increase in the manufacturing cost of the motor-controlling circuit.